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49 Cards in this Set
- Front
- Back
- 3rd side (hint)
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Doppler frequency / shift |
Increase in pitch on approach and decrease in pitch on departure. Frequency of sound changes when sound source & receiver move CLOSER together or FARTHER apart. |
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Demodulation |
Process of extracting the low doppler frequency from transducers carrier frequency. |
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Doppler shift created by ... |
Transmitted sound waves striking moving RBC. |
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Positive Doppler shift |
Blood cells move toward the transducer. Reflected frequency higher than transmitted frequency. |
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Negative Doppler shift |
Blood cells move away from transducer. reflected frequency lower than transmitted frequency. |
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Speed |
Magnitude only. Indicates distance a RBC moves in 1 second. |
distance ÷ time (cm/s) EX/ 50 miles/hour |
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Velocity |
Magnitude & direction. Think length of travel indicates magnitude, and angle indicates direction. |
EX/ 50 miles/hour WEST |
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Doppler Equation |
2 x velocity of blood x transducer frequency x cos angle ÷ propagation speed |
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Velocity of blood and Doppler shift |
Directly related. Faster velocity = greater doppler frequency. |
↓↓ or ↑↑ |
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X-axis of Doppler system represents ... |
Time |
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Y-axis of Doppler system represents ... |
Velocity |
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Frequency of transmitted sound and Doppler shift ... |
Directly related. If transducer freq. doubled = Doppler shift doubled. |
↓↓ or ↑↑ |
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Doppler shift measured in Hertz |
* directly related to velocity * directly related to transducer frequency |
↓↓ or ↑↑ |
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Sound beam direction vs. flow direction |
Parallel for entire velocity to be measured. Directly toward or away transducer for 100% accuracy. |
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Percentage of true velocity depends on ... |
Cosine of the angle between sound beam and direction of motion (flow). |
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Angle (degree) * 0° * 60° * 90° |
Cosine * 1 * 0.5 * 0 |
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Only portion of true velocity returned at angles other than ... |
0° and 180° |
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Bidirectional Doppler |
distinguishes direction of flow toward or away from transducer. |
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Continuous Wave Doppler |
2 crystals in transducer. One that constantly sends and one that constantly receives ultrasound energy. |
→ ← |
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Advantage of CW Doppler |
Ability to accurately measure very high velocities. |
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Primary disadvantage of CW Doppler |
Exact location of moving RBC cannot be determined. (Range ambiguity) |
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Dedicated CW transducer |
No anatomic image can be produced (only sound) . Uses NO backing material which makes it high quality & highly sensitive |
EX/ Pedoff |
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Pulsed Wave Doppler |
Only 1 crystal that alternates between sending & receiving ultrasound pulses. |
→ then ← |
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Advantage of PW Doppler |
Ability to select exact location to measure velocities. (Range specificity) |
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Primary disadvantage of PW Doppler |
Inaccurate measurement of high velocity signals. (aliasing) |
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Aliasing |
(False Identity) the most common error associated with Doppler ultrasound. Only when PW is used. |
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Aliasing spectral Doppler display |
RBC velocities reach top of spectral display & wrap to appear at the bottom.
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Nyquist Frequency / limit |
the highest Doppler freq. or velocity that can be measured without appearance of aliasing. (shown at the top of spectral display) |
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Nyquist formula |
Nyquist (Hz) = PRF (Hz) ÷ 2 |
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2 ways to avoid aliasing |
* raise the nyquist limit * reduce Doppler shift (Adjust: scale/sample depth/transducer freq./baseline) |
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Less aliasing ... |
* slower blood velocity * lower frequency transducer * shallow gate (high PRF) |
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More aliasing ... |
* faster blood velocity * higher freq. transducer * deep gate (low PRF) |
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Gray shades on Doppler spectrum are related to ... |
* amplitude on reflected signal * number of RBC creating reflection |
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Color Doppler measures MEAN (average) velocity |
Spectral Doppler (PW/CW) measures PEAK velocity |
Color vs. Spectral |
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Color Map (look-up table) |
Converts measured velocities into colors that appear on image. Displayed as verticle bar with black as center region. |
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2 most common used color map |
* velocity mode * variance mode |
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Velocity mode |
Info on direction & velocity. Above black = toward / positive Below black = away / negative Multicolor closer to black = slow Multicolor farther off black = fast |
Color changes always up or down, never side to side |
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Variance mode |
Indicates laminar or turbulent flow by color location on map. Left side = laminar flow Right side = turbulent flow (above/below black same toward/away) |
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Power Doppler (energy mode) |
Non-directional color doppler, only shows presence of a shift. |
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3 advantages of power mode |
* increased sensitivity to low flow. Venous flow/small vessels. * unaffected by angles, except 90°. * no aliasing, since velocity is ignored. |
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3 disadvantages power mode |
* no velocity or direction measurement. * lower frame rates, reduced temporal resolution. * affected by motion of patient, tissues, & transducer. |
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2 Doppler artifacts |
Clutter = low frequency Doppler shift artifacts. Ghosting = with color Doppler. |
Scary mess |
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Crosstalk |
Special form "mirror image" artifact, arises only from spectral Doppler. Identical above & below baseline. |
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Spectral analysis |
Tool that breaks the complex signal into basic blocks & identifies each velocity that makes up reflected signal. |
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2 methods of spectral analysis |
* fast Fourier transform (FFT) * autocorrelation |
Four fast autos |
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FFT |
Digital technique used to process both PW & CW. |
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2 advantages of FFT |
* exceedingly accurate * displays all individual velocities that make up reflected signal. |
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Autocorrelation |
Digital technique used to analyze color flow Doppler. |
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Helpful summary |
Doppler modes |
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